1. Field of the Invention
The present invention relates to a process and to a device for regenerating a fuel vapor filter for a direct injection engine.
2. Description of the Related Art
It is already known practice for motor vehicles to be fitted with fuel vapor filters. These filters are intended to collect and store the fuel vapors that are present in the vehicle tank. There are numerous factors which may be involved in creating these vapors--for example, the fact that the vehicle tank is partially empty and that the vehicle is standing in full sunshine. These vapors may alternatively be due to the fact that when the vehicle is in operation, excess fuel is sent to the injection line set. This excess fuel heats up in contact with the engine and then returns to the tank. Its entry into the tank creates fuel vapors.
The standards currently in force demand that vehicles must not emit fuel vapors. For this purpose, an active charcoal filter is associated with the tank so as to collect any fuel vapors. Of course, for this filter to operate in the optimum way, it has to be regenerated when saturated.
The purpose of the present invention is to regenerate such a filter installed on a direct injection engine.
When this filter is installed on a conventional indirect injection engine, the fuel vapors are conveyed to the inlet port upstream of the cylinders. Here, these vapors mix with the air and fuel injected and are then drawn into the cylinders when the inlet valves open.
By contrast, when such a fuel vapor filter is installed on a direct injection engine, a problem in introducing the fuel vapors into the cylinders arises since the combustion process differs.
For a direct injection engine to operate at optimum efficiency, it needs to operate as often as possible with what is known as stratified charge. In this type of combustion, the cylinder is filled with air when the inlet valve opens. Fuel is injected into the cylinder in the region of the spark plug so as to encourage the mixture to detonate at this point. However, as the distance from the spark plug increases, the richness of the mixture decreases until only air is present. There are thus a great many layers or "strata" of different richnesses within the cylinder. The layers closest to the spark plug are rich in fuel; the layers further away consist of what is known as a lean mixture. Stratified charge combustion makes it possible to ensure that the mixture will detonate and makes it possible to ensure, that as far as the cylinder as a whole is concerned, there is an excess of air, which greatly reduces the emissions of pollutants. Furthermore, this type of combustion makes it possible to minimize the amounts of fuel employed, and therefore to decrease the fuel consumption of the engine.
Now, for an engine to operate correctly on stratified charge, it is necessary for the inlet throttle valve to be opened as wide as possible so as to be able to operate with excess air. As a result of this, the inlet depression is very small, and this causes complications in conveying the fuel vapors toward the cylinders. The problem is that with no inlet depression, the fuel vapors are not correctly drawn in toward the cylinders. There is even a risk that these vapors might leave the vehicle through the air inlet pipe. Such a process would lead to even greater pollution, which would go completely against the desired objective.
European Patent Application EP 0 488 254 (TOYOTA) relates to an internal combustion engine of the direct injection type. This engine comprises a fuel vapor filter. This filter is regenerated when the engine is under heavy load and is running with a homogeneous charge in all cylinders. No regeneration is performed when the engine is running on stratified charge or at light load or when the catalytic converter has not reached its operating temperature (at start-up). What this means is that the filter is not regenerated at the appropriate instant (particularly on start-up because, throughout the time that the engine is not running, large quantities of vapor may accumulate in the filter). This document suggests no satisfactory solution for regenerating the fuel vapor filter during operation in stratified charge mode.